The present invention pertains to a sheet feed method for use in a sheet feeder having a sheet stacking unit provided with a bottom plate to support a sheet bundle whenever air is blown against a side edge of the sheet bundle near the topmost sheets of the bundle, thereby lifting these sheets from the bundle, separating the uppermost sheet from the bundle, and conveying this sheet away from the bundle. The present invention also pertains a sheet feeder and an imaging apparatus incorporating the sheet feeder.
A sheet feed method is known from US patent application 2004/0089994 A1. This method has been devised in order to be able and reliably feed a very broad range of recording media from the same sheet stacking unit. Nowadays, in printing rooms there are growing requests to form images on cardboard, tracing paper, and all sorts of coated media, etc. Many of such media have very smooth surfaces and with sheet feeding methods based on friction, mis-feeds and double-feeds occasionally occur. In order to mitigate these problems, known methods handle the sheet feeding by first blowing air from a side edge of a paper bundle, causing the uppermost sheets to lift off of the bundle. Then, air is injected between the uppermost and adjacent sheet, providing a very reliable separation of the uppermost sheet from the bundle. The actual separation is effected by using a suction unit, followed by conveying the sheet on a conveying belt. Over the friction based methods, the air separation methods has the advantage of a wide latitude of paper feed setting conditions, combined with the adaptability for high-speed processing, high durability and corresponding low running costs.
The known method however has an important disadvantage. It appears that the reliability of the feeding process decreases significantly when the bundle is nearly depleted, i.e., when less than 25 sheets of receiving media are present, particularly when less than 10 sheets are present. This is not restricted to extraordinary heavy or light media types. For example, with all sorts of plain paper, when less than 5 sheets are present, a mis-feed occasionally occurs. For the lighter types, typically types of less than 100 grams/square meter, the risk of inducing skew increases significantly when less than 5 to 10 sheets are present. Heavier types of media seem to be prone to mis-feeds, in particular when narrow paper formats (SEF) are being used. In order to overcome this problem, it is proposed to leave the last few sheets, typically 25-50 sheets, in the tray and then denote the tray as “empty”. These left-over sheets however, have to be removed from time to time, or immediately when another media type is going to be loaded in the sheet stacking unit. This prior art solution therefore is far from ideal.